In an earlier apparatus of this type, the control means generally was actuated by hand or included mechanical sensors on the walking props which responded to advance of the mining machine and/or the conveyor unit. Information for the advance of the walking props was obtained by, for example, the conveyor device to which the mining machine could also be attached, e.g. as a coal plow on the conveyor chian. It was also known to provide special sequence control which would automatically advance the walking props based upon a projection as to the actual advance of the mining face. With respect to the manually controlled systems, since the intervention of an operator was always necessary, there was a certain lack of precision in the advance of the walking props resulting from human intervention. The other systems were complex to construct and required expensive control equipment. A commonly assigned U.S. Pat. No. 4,900,091 is relevant to the advance of an array of walking props to follow the excavation of a mining face.